Iron-containing porcelain enamels for self-cleaning cooking oven

ABSTRACT

A porcelain enamel frit, containing a relatively high level of the oxide equivalent of iron, said frit adaptable to be incorporated into a vitreous porcelain enamel for application to an oven liner, said enamel characterized by the ability to oxidize oven soils when heated to a point above 350*F., but below 600*F., the method of utilizing same as a self-cleaning oven liner coating, and an oven liner coated therewith.

United States Patent Borowski 1 *Sept. 18, 1973 IRON-CONTAINING PORCELAIN ENAMELS [56] References Cited FOR SELF-CLEANING COOKING OVEN UNITED STATES PATENTS [75] Inventor: Bernard J. Borowski, Cleveland, 3,671,278 6/1972 Borowski 106/48 Ohio Primary Examiner-Edward G. Favors [73] Assignee. Ferro Corporation, Cleveland, Ohio Atmmey Mmon L Simmons [*1 Notice: The portion of the term of the patent subsequent t0 June 27, has been disclaimed. [22] Filed: 21 1972 A porcelain enamel frit, containing a relatively high level of the oxide equivalent of iron, said frit adaptable PP 232,267 to be incorporated into a vitreous porcelain enamel for Related Appncafion Data application to an oven liner, said enamel characterized I by the ability to oxidize oven soils when heated to a of 1970' Pat 3 671 278' utilizing same as a self-cleaning oven lincr coating, and

' I an oven liner coated therewith.

U.S. Cl- R, 6 Claims N0 Drawings [51] Int. Cl A2lb 1/00 [58] Field of Search 126/19; 106/48 IRON-CONTAINING PORCELAIN ENAMELS FOR SELF-CLEANING COOKING OVEN This is a continuation-in-part application of copending application Ser. No. 234,438 filed Mar. 13, 1972, which in turn is a divisional application of Ser. No. 43,97 1 filed June 5, 1970, since issued as US. Pat. No. 3,671,278 on June 20, 1972.

BACKGROUND OF THE INVENTION The field of this invention may be described generally as residing in coatings for oven liners, and more particularly, to an improved porcelain enamel frit of such a composition that upon being incorporated into a porcelain enamel coating and applied to a suitable substrate, then heated to within a critical temperature range, food soils, commonly spattered on an oven liner during baking or broiling, are readily and completely oxidized to a fine ash residue which is easily removable from the oven enclosure.

Whereas, heretofore, self-cleaning ovens were found generally in electric ranges, the enamel of this invention provides a means for making gas ranges self cleaning. That is, an electric range is, by design, capable of having self-cleaning portions thereof heated to high oxidation temperatures directly by elements placed along the reverse surfaces of the surfaces to be cleaned. A gas oven, on the other hand, could not be readily or practically designed so as to controllably elevate selfcleaning oven walls to oxidation temperatures by placing gas jets along the reverse surfaces thereof.

Accordingly, in order to effectively achieve a selfcleaning gas oven, it is essential that the interior, operating surface thereof be such that it will function at relatively low oxidation temperatures, and be capable of being brought up to those temperatures within an ambient oven atmosphere, utilizing standard oven baking or broiling burners conventionally positioned.

DESCRIPTION OF THE INVENTION It is well known that an increase in oxygen in an atmosphere effectively reduces the temperature at which materials are oxidized. This is particularly noted when organic materials are exposed to atmospheres having a high percentage of oxygen. I have found that certain porcelain enamels produced from frits (glass), effectively supply oxygen when organic materials, such as food soils, are in contact with the enamel surface, and that oxygen from a normal atmosphere is absorbed by the coating, which, it is theorized, is either stored or diffused throughout the enamel coating, thereby making available sufficient excess oxygen to promote the oxidation of food soils when the coating is subsequently heated in accordance with this invention.

This oxygen absorption phenomenon seems to occur at about the same surface temperture as needed for oxidation of an organic soil material contacting the surface, and is greatly enhanced by the presence within, and homogeneously smelted throughout, the enamel frit, of iron oxide.

At this point, a review of US. Pat. No. 3,266,477 to Stiles will aid materially in an appreciation of the unexpected and startling results of the instant invention.

Stiles teaches the use of a number of catalyst materials dispersed on oven surfaces via a number of different media for the purpose of aiding in the oxidation of oven soils.

From the general teaching of Stiles, it will be apparent that it is critical to his invention that his catalyst materials be thinly distributed on the oven surface.

One of the methods by which Stiles achieves a thin distribution of his catalyst on an oven liner is by sticking same to the surface thereof, Stiles quite emphatically stating, at the bottom of Column 2, lines -72 that, although he may distribute his catalyst material on the surface of an enamel coating and reheat the coating in order to "partially" embed his catalyst in the ceramic layer, that the catalyst should not be so deeply embedded as to obstruct access to his surface distributed catalyst. See also Column 5, lines 25 et seq. wherein Stiles again stresses that it is absolutely essential that his catalyst particles do not become too deeply embedded in the enamel coating as cleaning inactivity would result.

Examples 4, and 10-18 inclusive of Stiles are specific embodiments of the utilization ofa porcelain enamel as a base for Stiles catalytic particles. Attention is particularly directed to the number of complex steps in each operation in order to achieve a workable catalytic surface layer on porcelain enamel.

By the very nature of Stiles exposed catalytic layer the so-called catalytic particles of Stiles are highly susceptible to being removed by abrasion, thereby materially reducing their effectiveness after a period of time in use.

However, applicant has found, surprisingly, and directly contrary to the teachings of Stiles, that a totally unexpected improvement is achieved by incorporating high levels of iron oxide in the frit by smelting same homogeneously into and throughout the fritted, glass matrix, which is later milled into a porcelain enamel and applied to an oven liner, requiring only Step 1 of Stiles enamel examples, thereby enabling me to completely eliminate Steps 2-5 of Stiles Examples 4, and l1-l8 inclusive, and Steps 2-6 inclusive of Stiles Example 10.

Too, by incorporating iron oxide into the glass matrix of my frit, it becomes fixed throughout the enamel coating in such a permanent manner that no amount of abrasion can ever remove it. That is, should the enamel coating wear or abrade with use, the homogeneous dispersion of oxidation inducing iron oxide throughout the entire enamel coating insures that there will always be present within the coating, at or near the surface thereof, a supply of oxidation inducing oxide for oxidation of food soils.

Both the foregoing major advantages achieved by virtue of the unexpected improvement, contrary to the teaching of the art, of literally melting and completely submerging iron oxide into and homogeneously throughout the matrix of the glass fn't, the basic component of a porcelain enamel oven coating.

Another advantage of incorporating iron oxide directly into the frit is that, since the oxide is going to be melted directly into the glass, highly refined grades thereof are not required, standard ceramic grade sufficing nicely.

Previously, vegetable oil spatters on an oven surface normally required temperatures of 800F. to effectively oxidize them. The glass, porcelain enamel, or glaze made according to this invention will effectively oxidize and remove spatters at 500F. with the time of exposure to heat being the same as previously required for oxidation at 800F. Oxidation at lower temperaturcs approaching 350 l. can be achieved with the frits of this invention, but occurs at a reduced rate.

SUMMARY Briefly stated, this invention relates to a greatly improved frit and method of utilizing same for producing a porcelain enamel oven lining coating, the vitreous frit component of which contains a total of preferably 25 to 55 weight percent, but which may vary, depending upon frit characteristics from about weight percent to about 70 weight percent of iron oxide. Said frit subsequently milled into a porcelain enamel slip for application to and firing on the metal substrate which forrns a cooking oven liner.

The general box-like structure of a cooking oven or broiler liner is so well lrnown to the public that no graphic description of same is deemed necessary, and these are generally made of steel coated with porcelain enamel. However, as it is well known that porcelain enamel may be applied to any number of metallic substrates such as aluminum, stainless steel, etc., it is therefore within the realm of this invention to have an oven liner produced of any metal capable of withstanding cooking oven temperatures, subsequently coated with an enamel containing the frit of this invention.

DESCRlPTlON in the practice of my invention, an anamel frit is batch weighed, mixed, smelted and quenched into flake or frit utilizing conventional batch materials and procedures, except for the novel amounts of iron oxide as hereinafter described.

As will be developed infra, the base composition of the frit, aside from the contained iron oxide, which is useful to the practice of this invention is not considered critical, and it is required only that the frit, when smelted with a given percentage of iron oxide, be of such a fusability, and have the required physical characteristics, to be adaptable to be applied to a metallic substrate and fired into an adherent, porcelain enamel coating having preferably a matte to semi-matte surface.

The tems matte and semi-matte" are well lrnown in the art of porcelain enarneiing, and no detailed, quantitative description thereof is deemed necessary, although, in the interest of precision and a ready understanding of the specification and claims, both matte and semi-matte", refer to a surface demonstrating a 45 specular gloss value of less than 5, using a Photo volt Corporation glossmeter model 610, following ASTM Standard Method of Test for 45 Specular Gloss of Ceramic Materials [ASTM Designation (3-3464? (Reapproved 1967)], with the preferred value being about 3 or less.

And, although composition as such is not important, below in Table i is listed, for general guidance, the oxide components which could go to make up any given frit useful to this invention, demonstrating a general range of each component by weight percent, it being understood that, as the weight percentage varies for the components of any given frit within the indicated range, they would total ltd-ll.

Furthermore, other than the Fe h range, which is the essence of this invention, one or more of the listed components may be completely eliminated in a given frit, or other well-known oxides, although not listed below, may be added for various special purposes.

TABLE l Frit Oxide Composition Parts by Weight Wt.% A1 6, e20 1-10 CaO 0-1 Co l 0-5 CuO 0-4 Kai 0-10 Les 0-5 w s O-lO P205 O-2 sio, 15-20 Ti!) 10-30 ZnO 0-4 ZrO O-l0 F0203 15-70 The foregoing components totalling 100 as they vary within the ranges indicated.

The quantity of the iron oxide used is dependent upon the other elements that make up the glass only to the degree that higher levels of iron oxide might vary the fusability of a given glass, and thereby alter its workability characteristics, requiring appropriate adjustment of the amount of iron oxide within their workable range.

And as stated above, although our preferred range of iron oxide is on the order of 25 to 55 weight percent of the frit, either singly or in combination, some recognizable, measurable oxidation properties can be achieved at as low as 15 weight percent, and as high as weight percent, depending upon the given characteristics of a given frit such as fusability, etc.

it is to be understood that there is no measurable upper limit of the oxidation inducing oxide, the only re quirement being that there be enough other glass forming components to provide a glass which is workable and has the general qualities of a porcelain enamel at conventional firing temperatures.

As an example of the possibility of eliminating an oxide set forth in Table l, although the preferred range of TiO; is 10 to 30, facilitating smelting of the glass, fair to good cleanability, following the procedures set forth infra, may be achieved at 5.0 weight percent TiO and fair cleanability at zero weight percent, although complete elimination of TiO does render the glass more difficult to smelt properly.

Nevertheless, elimination of Ti0 completely does demonstrate the degree of flexibility possible with the balance of the glass matrix, so long as the Fe O level is maintained, and a workable glass, from both smelting and application standpoint is achieved, capable of providing a self-cleaning, matte to semi-matte surface.

Exemplary of specific vitreous enamel compositions useful in the practice of this invention are the following specific working examples.

EXAMPLE ii The following composition was weighed and mixed in a suitable raw batch mixer, or blender.

Parts by Wt. l32

Copper Oxide The foregoing mixture was then smelted at 2550F., quenched (fritted) in cold water, and dried in a drier at 200F., the resultant frit having the following oxide weight percent composition:

Oxide Weight Percent The resulting frit was then milled in a conventional ball mill using the following mill addition.

Wt.% Glass (frit) 100 Polytran FS' is NaNO, 75 Water 45 'POLYTRAN FS is a water-soluble bio-polymer composition containing scleroglucan, a high molecular weight poly-Saccharide produced by fermentation. The polymer structure is substantially a linear chain of anhydroglucose units linked beta l-3. Thirty to thirty-five percent of the linear chain units bear single appended anhydroglucose units linked beta 1-6.

This enamel was milled to a fineness of i grams/400 mesh/50 cc., then sprayed on a suitably prepared substrate at an application weight of 30 gms/sq. ft. The enamel was then fired at l450F. for three minutes to a matte surface, and the enamels of this invention will normally be fired at temperatures lower than 1600F.

The foregoing mill addition is, as will be readily apparent, considered to be clayless, and is applicable to either a ground coated metallic substrate, or by a direct-on process, to a suitably prepared metal substrate.

The frit containing the oxidation inducing iron oxide above was compared to a similar frit which contained no such oxide, as well as to a conventional oven enamel in the following test. In this and all subsequent examples, the standard of comparison was a blank enamel having substantially the same composition as that enamel of this invention being evaluated, except for the oxidation inducing iron oxide, along with a conventional oven enamel found in oven ranges heretofore, and which contained no oxidation inducing iron oxide at the levels utilized in the frit of this invention.

This and the following examples were evaluated by heating to 390F., at which temperature one drop each of various types of food soils (shortening, meat renderings and sugar water) was placed on the hot sample with an eye-dropper. The test I samples were then heated at 550F. for two hours.

At the end of this two hour period the samples were cooled and evaluated. The coating of this invention with the irondioxide smelted in had left no residual stain, while the two comparison standards containing no oxidation inducing iron oxide, had a tightly adher ing, unsightly black residue where the food samples had been placed.

EXAMPLE 2 The following composition was weighed and mixed in a blender.

Parts by Wt. Borax 56 Soda Ash l58 Sodium Nitrate 34 Calcined Alumina I82 Powdered Quartz 418 Red Iron Oxide l664.5 Rutile 287 The foregoing mixture was then smelted at 2550F., quenched (fritted) in cold water, and dried in a drier at 200F., the resultant frit having the following oxide weight percent composition:

Oxide Weight Percent 510, 15.52 5,0, 1.35 Na,0 4.49 A1,o, 6.68 131,0, 6|.74 T10, 10.22

The frit was milled in a conventional ball mill using the following mill addition:

Wt.% Glass (Frit) Boric Acid 5.0 Bentonite 3/8 Gurn Tragacanth l/l6 Powdered Aluminum 62.5 Water 32 This enamel was milled to a fineness of 10 grams/400 mesh/50 cc., and sprayed directly onto a suitable metallic substrate at an application weight of 30 gms/sq. ft., then fired at l400F. for three minutes to obtain a matte surface.

The foregoing frit was evaluated to a blank standard, and a conventional oven enamel according to the procedures of Example 1. At the end of this two hour test period the samples were cooled and compared. The glass with the smelted-in iron oxide had no residual stain, while the sample of a conventional enamel, and the standard minus iron had a tightly adhering black residue.

EXAMPLE 3 A glass with the following raw batch composition was batch weighed and mixed in a blender.

The mixture was then smelted at 2550F. for 40 minutes, quenched in cold water and dried in a drier at 200F., the resultant frit having the following oxide weight percent composition:

Oxide Weight Percent SiO, 32.72 B 0 2.99 Na,O 1.70 K,O 5.26

Ll,O 0.79 CaO 0.40 g s 0.60 is it; n F6 35.70

The frit was then milled in a conventional ball mill using the following mill addition:

Glass (Frit) 100 Polytran A NaNO, /6 Water 45 This enamel was milled to a fineness of 10 grams/400 mesh/50 cc., then sprayed on a suitably prepared substrate at an application weight of 30 gms/sq. ft., then fired at 1450F. for three minutes.

The samples were evaluated following the same procedure as in Example 1. At the end of the two hour test period the samples were cooled and evaluated. The glass with the smelted-in iron oxide had no residual stain, while samples of the conventional enamel and the blank standard had tightly adhering black residues where the food samples had been applied.

EXAMPLE 4 The following composition was weighed and mixed in a blender.

Parts by Wt. Borax 28 Boric Acid l Potassium Carbonate 202 Sodium Nitrate 73 Potters Whiting 19 Sodium Tripoly Phosphate 27 Lithium Carbonate 52 Milled Zircon 93 Powdered Quartz 820 Copper Oxide 92.7 Ruti e 362 Red lron Oxide 927 This glass was then smelted at 2550F. for 50 minutes, quenched in cold water and dried in a drier at 200F., the resultant frit having the following oxide weight percent composition:

Oxide Weight Percent SiO 32.85 8,0 2.99 Na O 1.79 K ,0 5.27 1.1,0 0.80 CaO 0.40 ZIO; 2.40 as 2-22 Ll TiO, 13.44 Fe O 35.86

The frit was then milled using the following mill addition:

Wt.% Glass (Frit) 100 Boric Acid 5.0 Bentonite 3/8 Gum Tra aczmth l/lb Powdere Aluminum 62.5 Water 32 This enamel was milled to a fineness of grams/400 mesh/50 00., then sprayed (direct on, no intermediate ground coat) onto a suitably prepared substrate at an application weight of 30 gms/sq. ft., then fired at l500F. for two minutes to obtain a matte surface.

These samples were evaluated using the same procedure as in Example 1. After the cleaning cycle the glass with smelted-in iron oxide was substantially free of residual stain, while samples of conventional enamel and the blank standard had a tightly adhering black deposit.

EXAMPLE 5 A glass with the following composition was weighed and mixed in a blender.

Parts by Wt. Borax I23 Potassium Carbonate I54 Rutile 450 Potteis Whiting 22 Sodium Tripoly Phosphate 27 Lithium Carbonate 54 Milled Zircon I00 Powdered Quartz 906 Potassium Nitrate 91 Red Iron Oxide 550 The mixture was then smelted at. 2550F. for minutes, quenched in cold water and dried in a drier at 200F., the resultant frit having the following oxide weight percent composition:

Oxide Weight lPercent The frit was then milled in a conventional ball mill using the following mill addition:

Wt.% Glass (Frit) I00 Polytran F S A NaNO, 5: Water This enamel was milled to a fineness of 10 grams/400 mesh/ 00., then sprayed on a suitable prepared substrate at an application weight of 30 gms/sq. ft. then fired at 14 0OF. for three minutes to obtain a matte surface.

The samples were evaluated following the same procedure as in Example 1. At the end of the two hour test period the samples were cooled and evaluated. This glass with l7 percent iron smelted in had left no residual stain while a sample of a conventional enamel had a tightly adhering black deposit.

As stated above, the composition of the frits useful to this invention is not overly critical and this fact will be supported by setting forth in Table ll below the approximate ranges of various oxide components spanned by Examples 1-5.

TABLE ll Frlt Oxide Composition Parts by Weight Wt.% Algog 0-15 B 0 1-3 CaO O-l Co,O= 0-4 CuO 0-4 14,0 06 Li O 0-l Na O 1-8 P,o, -l 16-40 TiO: -20 ZnO 0-4 ZrO 04 m6, l7-65 The foregoing components totalling 100 as they vary within the ranges indicated.

While I have demonstrated a utility for this class of vitreous coatings in connection with self-cleaning cooking ovens, it is to be understood that the ability of a vitreous coating such as mine to oxidize organic compounds would also have wide application in a number of different fields, for example, utilization of my enamel in connection with exhaust systems for paint baking ovens whereby the vapors, prior to being discharged into the open atmosphere, would be exposed to a series of heated surfaces covered with my porcelain enamel and thereby oxidized to unobjectionable compounds from the standpoint of air pollution. The same theory would apply to automotive exhaust systems, the interiors of which could be coated with my enamel to render certain products of combustion less noxious and objectionable prior to their discharge into the atmosphere.

The ability of these coatings to oxidize organics is a function of time, temperature and the type of organic to be oxidized. Generally little or no effective oxidation can be achieved below a temperature of 350F. It is contemplated that the upper temperature limit of oxidation utilizing my coating would be in the vicinity of 600F. This providing the added advantage that extensive safety and locking devices are not considered necessary at this temperature, inasmuch as the likelihood of an explosion resulting from the sudden availability of excess oxygen due to inadvertent opening of the oven door is considered to be materially reduced within this range whereas, at the higher temperatures required for previous methods of heat oxidation of oven soils, there was always the ever present danger of such explosion, requiring the inclusion in the appliance of a safety locking device for the oven door, to be utilized during the oxidation process.

From the foregoing working examples, except as the percentage of oxidation inducing oxide in the frit may be reduced by mill added components, such as alumina in Example 4, the weight percent of the oxidation inducing oxide will occur in the final, fired, enamel coating in substantially the same weight percent as in the frit.

Having thus described and illustrated my invention, it is set forth in the following claims which are to be construed in the light of the United States statutes and decisions in such a manner as to give them the broad range of equivalents to which they are entitled.

I therefore claim:

1. A metallic cooking oven liner having fused to its operating surface at a temperature less than 1600F., a porcelain enamel coating, having a matte to semi-matte surface, said coating composed of a frit having had smelted homogeneously therethroughout from about to about 70 weight percent, based on the weight of said vitreous frit composition, of the oxide equivalent of iron.

2. The oven of claim 1 wherein the total of said oxide equivalent of iron is in said coating in from about 25 to about 55 weight percent based on the weight of said frit.

3. A metallic cooking oven liner having fused to its operating surface at a temperature less than 1600F. a porcelain enamel coating having a matte to semi-matte surface, adaptable to oxidize organic soils thereon when heated to a temperature of from about 350F. to about 600F., said coating composed of a frit characterized by having had smelted homogenously therethroughout from about l5 to about weight percent, based on the weight of said vitreous frit composition, of the oxide equivalent of iron, the balance of said frit having had at least five additional oxide equivalents smelted therein selected from the following, and present within the range of each respectively:

\Vt.% ALO; 0-20 8,0; ll 0 CaO O-l C0 0, 0-5 CuO 0-4 x ,0 ol 0 L|,O 0-5 M1 0 0- l 0 P ,O, 0-2 S10, 15-20 TiO, 10-30 ZnO 0-4 ZrO, 0- l O Fe,0 15-70 the foregoing totalling as they vary within the range indicated.

4. The oven of claim 3 wherein the total of said oxide equivalent of iron is in said frit in from about 25 to about 55 weight percent.

5. A metallic cooking oven liner having fused to its operating surface at a temperature less than 1600F. a porcelain enamel coating having a matte to semi-matte surface, adaptable to oxidize organic soils thereon when heated to a temperature of from about 350F. to about 600F., said coating composed of a frit characterized by having had smelted homogeneously therethroughout from about 15 to about 70 weight percent, based on the weight of said vitreous frit composition, of the oxide equivalent of iron, the balance of said frit having had at least five additional oxide equivalents smelted therein selected from the following, and present within the range of each respectively:

AMO O- l 5 i s 1-3 CaO 0 1 Co,0, 0-4 CuO 0.4 K ,O 0-6 L 0-l Na,0 l-8 s s 0-l S O, l6-40 T10, l0-20 gnrg 0-4 1 0-3 Fe,0, 17-65 the foregoing totalling 100 as they vary within the range indicated.

6. The oven of claim 5 wherein the total of said oxide equivalent of iron is in said frit in from about 25 to about 55 weight percent.

# t i i 

2. The oven of claim 1 wherein the total of said oxide equivalent of iron is in said coating in from about 25 to about 55 weight percent based on the weight of said frit.
 3. A metallic cooking oven liner having fused to its operating surface at a temperature less than 1600*F. a porcelain enamel coating having a matte to semi-matte surface, adaptable to oxidize organic soils thereon when heated to a temperature of from about 350*F. to about 600*F., said coating composed of a frit characterized by having had smelted homogenously therethroughout from about 15 to about 70 weight percent, based on the weight of said vitreous frit composition, of the oxide equivalent of iron, the balance of said frit having had at least five additional oxide equivalents smelted therein selected from the following, and present within the range of each respectively: Wt.% Al2O30-20 B2O31-10 CaO0-1 Co2O30-5 CuO0-4 K2O0-10 Li2O0-5 Na2O0-10 P2O50-2 SiO215-20 TiO210-30 ZnO0-4 ZrO20-10 Fe2O315-70 the foregoing totalling 100 as they vary within the range indicated.
 4. The oven of claim 3 wherein the total of said oxide equivalent of iron is in said frit in from about 25 to about 55 weight percent.
 5. A metallic cooking oven liner having fused to its operating surface at a temperature less than 1600*F. a porcelain enamel coating having a matte to semi-matte surface, adaptable to oxidize organic soils thereon when heated to a temperature of from about 350*F. to about 600*F., said coating composed of a frit characterized by having had smelted homogeneously therethroughout from about 15 to about 70 weight percent, based on the weight of said vitreous frit composition, of the oxide equivalent of iron, the balance of said frit having had at least five additional oxide equivalents smelted therein selected from the following, and present within the range of each respectively: Wt.% Al2O30-15 B2O31-3 CaO0-1 Co2O30-4 CuO0-4 K2O0-6 Li2O0-1 Na2O1-8 P2O50-1 SiO216-40 TiO210-20 ZnO0-4 ZrO2 0-3 Fe2O317-65 the foregoing totalling 100 as they vary within the range indicated.
 6. The oven of claim 5 wherein the total of said oxide equivalent of iron is in said frit in from about 25 to about 55 weight percent. 